The present invention refers generally to a system comprising an evaporator and in particular to an evaporator in the form of a plate heat exchanger. Generally, an evaporator is designed for evaporation of a fluid, such as a cooling agent, for various applications, such as air conditioning, cooling systems, heat pump systems, etc. Thus the evaporator may be used in a two-phase system handling a fluid in a liquid form as well as in a gaseous or evaporated form.
In case of the evaporator being a plate heat exchanger, this may by way of example include a plate package, which includes a number of first and second heat exchanger plates. The plates are permanently joined to each other and arranged side by side in such a way that a first plate interspace, forming a first fluid passage, is formed between each pair of adjacent first heat exchanger plates and second heat exchanger plates, and a second plate interspace, forming a second fluid passage, between each pair of adjacent second heat exchanger plates and first heat exchanger plates. The first plate interspaces and the second plate interspaces are separated from each other and provided side by side in an alternating order in the plate package. Substantially each heat exchanger plate has at least a first porthole and a second porthole, wherein the first portholes form a first inlet channel to the first plate interspaces and the second portholes form a first outlet channel from the first plate interspaces and wherein the plate package includes a separate space for each of said first plate interspaces, which space is closed to the second plate interspaces.
In this general prior art plate heat exchanger to be used in a two-phase system a first fluid, such as a cooling agent, is introduced into the valve in liquid form but expands when going through the valve due to the pressure drop into a partly evaporated fluid at one end of the first inlet channel, i.e. the first port hole, for further distribution along the first inlet channel and further into each of the individual first plate interspaces during evaporation into an evaporated form. There is always a risk that the energy content of the supplied fluid is too high, whereby a part of the flow supplied to the inlet channel via its inlet port will meet the rear end of the inlet channel and be reflected thereby in the opposite direction. Thereby the flow in the inlet channel is very chaotic and hard to predict and control.
Further, the pressure drop of the cooling agent may increase with the distance from the inlet to the first inlet channel, whereby the distribution of the first fluid between the individual plate interspaces will be affected. It is known that the angular flow change that the droplets of the first fluid must undergo when entering the individual plate interspaces from the first inlet channel contributes to an uneven distribution. Yet another influencing parameter is dimensional differences between the individual first plate interspaces, resulting in that each first plate interspace has its unique efficiency. It is also to be known that the operation and performance of an individual first plate interspace depends on its position in a plate package. The outer most first plate interspaces on each side of the plate package tend to behave different than those in the middle of the plate package.
As a result of this it is very hard, or even impossible, to optimize the operation and efficiency of an evaporator as a whole, ensuring that all fluid supplied to the evaporator is fully evaporated before leaving the outlet of the evaporator and especially before reaching the inlet of a compressor to be arranged downstream of the outlet of the evaporator. In fact it is sufficient that there is one malfunctioning first plate interspace for insufficient evaporation of the evaporator as a whole to occur. By way of example, if one single first plate interspace is flooded, i.e. is incapable of evaporating the complete amount of fluid supplied thereto, droplets will occur downstream the outlet of the evaporator. Generally, by fully evaporated means that the evaporated fluid must have reached a superheating temperature difference whereby the evaporated fluid comprises dry evaporated fluid only, i.e. the evaporated fluid should have a temperature being higher than the saturation temperature at a prevailing pressure.
The purpose of operating the evaporator as close to a superheating set-point temperature as possible no matter operation duty is of importance to get as high utilization factor as possible. Thus, it is of economic importance. Further, it has an influence to other components cooperating with the evaporator, such as a compressor, since compressors normally are sensitive to liquid content. Any droplets remaining in the evaporated fluid when reaching the inlet of the compressor may damage the same. Also, there is an economical interest of operating the evaporator as close to the superheating temperature difference as possible since once the fluid has reached the superheating temperature difference the fluid is completely dry and there is no substantial gain in increasing the temperature additionally. The superheating temperature set-point above is determined by the system manufacturer to incorporate a certain wanted safety margin against the risk of receiving liquid into the compressor. The problems discussed above get more pronounced when the load of the evaporator is changed. This may by way of example be the case when changing the operation duty of an air conditioning system, from one temperature to another, meaning that the amount of fluid to be supplied to the evaporator is changed.
Documents EP2156112B1 and WO2008151639A1 provide a method for controlling a refrigerant distribution among at least two evaporators in such a manner that the refrigeration capacity of air-heated evaporators is utilized to the greatest possible extent. This is made by monitoring a superheat of refrigerant at a common outlet of the evaporators. Further, this is made by altering a mass flow of refrigerant through a selected evaporator while keeping the total mass flow of refrigerant through all the evaporators substantially constant. The flow is controlled by one single valve being an expansion valve. Thus, the two documents provide a solution to controlling the operation of a plurality of air-heated evaporators, in which method each evaporator is evaluated as a complete unit and in which method each unit is controlled in view of additional evaporators arranged in the same circuit.
Other examples of documents disclosing systems comprising multiple evaporators and/or multiple heat exchangers are U.S. Pat. No. 6,415,519B1 and EP0750166A2. In U.S. Pat. No. 6,415,519B1, multiple evaporators are utilized for cooling a multi-component computer system. In EP0750166A2, a plurality of indoor heat-exchangers is disclosed. Also these two documents provide solutions to controlling the operation of a plurality of heat-exchangers and/or evaporators in a system, in which each evaporator/heat-exchanger is evaluated as a complete unit.
Generally, the efficiency of evaporators and especially plate heat exchangers at part load is a raising issue. More focus is put on how the evaporator performs at different operation duties instead of being measured at only one operation duty. By way of example, laboratory scale trials have shown that an air-conditioning system can save 4-10% of its energy consumption just by improved evaporator function at part load for a given brazed plate heat exchanger. Further, an evaporator system is typically only operating at full capacity for 3% of the time, while most evaporators are designed and tuned for a full capacity operation.